The specific hypothesis that the central nervous system control of locomotion is focused on distinct parts of individual muscles rather than whole muscles or synergistic groups will be tested by recording the activity of single motor units in distinct parts of the triceps surae muscle group of cats during a broad range of stepping behaviors. Each motor unit recorded will be carefully characterized using physiological criteria, and its location in muscle examined by glycogen depletion and histochemical analysis. The pattern of recruitment of motor units in each part of each of the muscles will be correlated with the physiological properties of those units. These patterns will then be compared between different parts of the muscle to determine if motor units are recruited with any level of independence in different parts of the same muscle. The results of this study are expected to have significance in establishing guidelines for studies of the mechanisms and the specificity of motor control during behavior. They are also anticipated to be of use to clinicians, especially in neurology and rehabilitation medicine, in the diagnosis, treatment and evaluation of patients with disorders of movement, especially those involving the locomotor apparatus.